[Bugfix] Return HTTP 500 when is_exist reports an error in handle_exist

[he-yufeng]

Problem

handle_exist used bool(is_exist(key)) to populate the response body.

is_exist() returns three distinct values:

Return	Meaning
0	key does not exist
positive integer	key exists
negative integer	store error

bool(-1) evaluates to True in Python, so a store error was silently reported as HTTP 200 {"exists": true} instead of HTTP 500. Callers have no way to distinguish "key found" from "store is unhealthy".

Fix

Mirror the pattern introduced for handle_get in #2587: check exists < 0 first and return HTTP 500 {"error": "Exist check failed"}. Also replace bool(exists) with exists > 0 to make the positive-vs-zero distinction explicit.

Test plan

New test test_handle_exist_store_error sets is_exist = lambda key: -1 and asserts the response is HTTP 500 with an "error" key.

To verify:

cd mooncake-wheel
python -m pytest tests/test_mooncake_store_service_api.py -k exist -v

All exist-related tests pass. Four unrelated tests (test_handle_put_missing_value, test_reconfigure_*, test_unmount_shm_string_segment_id_coercion) fail on upstream main as existing failures before this change and are unaffected.

Relation to #2587

#2587 fixed the same class of bug in handle_get (which also calls is_exist and previously used if not value as a truthiness check). This PR applies the same correction to the handle_exist endpoint.

[gemini-code-assist]

Code Review

This pull request updates the handle_exist method in mooncake_store_service.py to properly handle store health check failures (indicated by a negative return value) by returning an HTTP 500 error instead of an HTTP 200 with a false positive. It also adds a corresponding unit test. The review feedback suggests improving the robustness of the new unit test by asserting the specific error message rather than just checking for the presence of the 'error' key.

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[gemini-code-assist]

The current assertion self.assertIn("error", body) only checks for the presence of the "error" key in the response. If an unexpected exception occurs in the try block of handle_exist, the except Exception block will catch it and return a 500 response with an "error" key containing the exception string. This would cause the test to pass even if the specific error path wasn't hit.

To make the test more robust and prevent false positives, assert the specific error message "Exist check failed" in body["error"], matching the pattern used in other tests like test_handle_get_exist_check_failure.

Suggested change

	body = json.loads(resp.text)
	self.assertIn("error", body)
	body = json.loads(resp.text)
	self.assertIn("Exist check failed", body["error"])

[Lin-z-w]

I agree with this suggestion. The test should assert the specific "Exist check failed" message so it verifies the new exists < 0 path rather than any generic 500 error response.

[he-yufeng]

Good call, done in 0365cd5. The assertion now checks body["error"] == "Exist check failed", so it pins the exists < 0 branch specifically and won't pass on the generic except path (which returns {"error": str(e)}).

[codecov-commenter]

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[Lin-z-w]

Thanks for the fix. LGTM.